It is well known in the art of electrographic and electrophotographic copying and printing to form an electrostatic latent image corresponding to either the original to be copied, or corresponding to the digitized data describing an electronically available image, on a photoconductive member. In another image forming method, the electrostatic latent image is formed by imagewise discharge over styli towards a dielectric substratum. The xeroprinting process such as disclosed e.g. in European Patent Application 0 243 934 involves imagewise exposing a photopolymer master, charging e.g. by corona, toning with dry or liquid toner and transferring to another substrate.
Electrostatic latent images can be developed using a liquid developer consisting of a colloidal system of charged colloidal particles in an insulating liquid. In most cases the latent image is developed with a finely divided developing material or toner to form a powder image which is then transferred onto a support sheet such as paper. The support sheet bearing the toner powder image is subsequently passed through a fusing apparatus and is thereafter discharged out of the copying resp. printing machine as a final copy, resp. final print.
One of the objectives set forth for the overall electrostatographic process is to provide an image on the final copy, resp. final print with the best possible quality.
By `quality` in electrostatography is generally understood a true, faithful reproduction of the original to be copied, or faithful visual print of the electronically available image.
Quality consequently comprises features such as uniform darkness of the image areas, background quality, clear delineation of lines, as well as overall resolution of the image.
The accuracy, inclusive of the resolution, by which the latent electrostatographic image, formed in either an electronic printing or copying apparatus, is developed into a visually discernable copy, is predominantly determined by the characteristics of the developer used.
It is known that one of the principal contributing characteristics herefore is the size and size distribution of the developer particles used, and in case a two component developer material is used, in particular the size and size distribution of the toner particles employed.
In the document published by ATR Corporation, 6256 Pleasant Valley Road, El Dorado, Calif. 95623, entitled `Effect of Toner Shape on Image Quality` published Mar. 28, 1988, the influence of toner particle diameter and shape upon image quality, particularly for high resolution images, has been tested.
For a developer to be used in a high resolution laser beam printer, the effects of toner particle size and shape upon the image were experimentally examined. As a result it was made clear that apart from the shape of the toner and its charge distribution, fine particles are most effective to provide high resolution.
This fact being known in itself, there have been several prior art proposals for the manufacture of fine toner particles and in particular for toner particles, the size distribution of which meets a well-defined classification.
In U.S. Pat. Nos. 3,942,979 and 4,284,701 the preparation and use of toner particles with well-defined particle-distributions has been disclosed.
Apart from the size distribution, the resolution of an electrostatographic print is determined also to a large extent by the average or median size of the toner particles. So efforts have been made to manufacture and use extremely fine developer materials.
However the smaller the average size of the toner particles, the greater the tendency of the toner and of the overall developer composition to agglomerate or to cluster together, and the greater the tendency of the toner particles to stick to the image bearing member. In order to partially remedy these problems it is known to add a lubricant, e.g. zinc stearate, in an amount of at least 0.5 wt %, to the developer composition, as is disclosed e.g. in UK application GB 2,180,948.
In JP KOKAI 60-159857 a magnetic toner superior in fluidity and tranferability is disclosed, said toner comprising a thermoplastic resin such as polyethylene wax, a colorant, a magnetic material, a silica and a surfactant having perfluoroalkyl groups.
In JP KOKAI 63-300243 a low-temperature fixable toner is disclosed wherein in order to prevent blocking the surface of each specified particle is coated with a specified F-containing silane compound.
EP-A-0 050 987 discloses a one-part flowable, dry, electrostatically attractable toner comprising a thermoplastic binder and a magnetically responsive material and a surface treatment composition comprising a fluoroaliphatic compound.
Also in JP KOKAI Nr. 60-93455 the addition of a fluorine substituted silane coupling agent to toner is disclosed, said agent being e.g. colloidal silicon dioxide. In said specification the effects of enhanced flowability and negative chargeability of the developer wherein such fluorinated particles are incorporated, are set forth and in particular the decreased dependence on environmental conditions is put forward. In the examples of said specification, a styrene-acrylic and a polyester resin is used as binder for the developer, whereas the particle size of the developer is set to be around 10 micron.
In EP-A-0 335 676 a developer for developing electrostatic images is disclosed, said developer comprising 0.05 to 3 wt parts of hydrophobic silica fine power having a tribo electric chargeability of -100 to -300 .mu.c/g. According to a preferred mode described in said specification the silica fine power is produced through vapor phase oxidation of a silicon halide, treated with a silane coupling agent and/or silicone oil. As preferred examples of such silicone oil are mentioned i.a. a fluorinemodified silicone oil.
It is quite common in the art of manufacturing electrostatographic developers to incorporate charge controlling agents (commonly called CCA's) into the developer mass. In the latter EP specification, it is set forth that the toner according to the invention disclosed therein may also contain as desired such CCA-compound, and in all (eight) examples described in said specification a negative charge control agent (a particular Cr-complex) is indeed incorporated in the developer.
A difficulty arising in the manufacture of fine toner particles is the requirement of intimate and uniform incorporation of the ingredients in the toner particles. Variations in the composition of the individual toner particles should be avoided. This applies in particular to the incorporation of CCA-compounds in the toner particles. Indeed, variations in the amount of CCA's incorporated in the toner particles give rise to varying charges carried by the toner particles which in turn cause several problems in processing e.g. a selectivity in development so that the composition of the developer in multi-use changes as a function of print amount, further a shift in average charge-mass relationship (Q/m) value altering the copy quality by variations in copy density, resolution etc. Concomitantly a detrimental effect on the transfer efficiency is observed. Such non-uniform distribution of the CCA's over fine toner particles in particular occurs when using large CCA-compounds, such as disclosed e.g. in U.S. Pat. No. 4,525,445. When using soluble CCA-compounds the uniformity of the distribution of the CCA-compounds over the toner can be increased, however such soluble CCA have a plasticizing effect on the toner particles, causing other problems such as a reduction of the lifetime of the toner particles by smearing on the carrier surface. This phenomenon impedes the use of tougher (higher Tg) polymers, indirectly affecting fusing characteristics in an adverse way. This causes difficulties in particular when toners/developers are considered for use in full-color applications. Finally one could use CCA-compounds built-in in the toner-binder resin. These CCA-compounds have the disadvantage however of a high price and complexity in preparation. As a conclusion it may be set forth that the problem-free incorporation of CCA-compounds in toner particles featuring a well-defined size distribution as characterized by e.g. a fine average particle size is not yet satisfactorily solved.